This invention relates to an acceleration sensor with a cantilevered bending beam, mounted in a housing, with the free end of the bending beam carrying a permanent magnet which functions both as inertial mass and as a signal generator. The beam is deflectable relative to the sensor housing in the plane of the acceleration to be measured. A measuring system for determining the beam deflection in response to an acceleration of the housing includes the permanent magnet(s) as well as one or more magnetic-field-sensitive position sensors (so-called elementary sensors) mounted in the housing, to produce a signal representative of the position or change in position of the permanent magnets.
An acceleration sensor of this general type is described in German Patent Specification (DE) No. 28 29 425 C2. In a closed housing, a seismic mass is mounted on a cantilevered leaf spring unilaterally clamped in the housing. The seismic mass is fixed on the free end of the leaf spring and consists of a moving iron part which, upon deflection, disturbs the magnetic field of a permanent magnet fixed to the housing on the level of the free end. Upon deflection of the mass, an electrical signal is generated by means of two magnetic-field-sensitive ohmic resistors, in particular magnetoresistors, arranged symmetrically to the rest position of the seismic mass and perpendicularly to the field lines of the magnetic field in the air gap between the permanent magnet or the permanent magnet's pole shoes and the seismic mass as well as perpendicularly to the direction of motion of the seismic mass. It is also possible to use a permanent magnet as the seismic mass instead of the moving iron part, in which case the iron parts are fixed to the housing.
A considerable manufacturing effort is required in order to gain a usable measured signal by means of such a device even in case of small accelerations. The forces required to deflect the leaf spring are relatively high which requires the use of a relatively large seismic mass. The selectivity of the deflection in the measuring direction leaves much to be desired. Therefore, such a sensor is not suitable for mass production such as for control engineering applications in automotive vehicles where generally a plurality of like sensors is needed and where the manufacturing price plays a decisive part.
German Patent Specification (DE) No. 31 33 056 C2 also discloses a bending beam sensor. Within a closed housing, there is arranged a bending beam in the form of a unilaterally clamped elastic arm of epoxy resin, vinyl chloride or beryllium copper fixed at one end to the housing. The free end of the elastic arm carries a weight. In the lower part, i.e., near the point where it is fastened to the housing, the elastic arm is surrounded by a coil. In that part of the arm lying within the coil, the arm carries a flat body of an amorphous low-retentivity metal. When the elastic arm is deflected by an acceleration or is caused to vibrate, the deformation will lead to a tensile or compressive load in the low-retentivity body, the load causing a corresponding electric signal at the coil. The signals are electronically transformed into acceleration or deceleration values. The expenditure for such sensors is also very high, particularly if there are demands for a high-degree of measuring accuracy and measuring sensitivity.
Further, a vehicular deceleration measuring device known from Published Patent Application (Offenlegungsschrift) (DE) No. 30 16 001 A1 has a unilaterally clamped leaf spring on the free end of which an inertial mass is mounted with a rod magnet being mounted on the inertial mass. The change of position of the rod magnet is determined by means of a Hall element. A second permanent magnet fastened to the housing is provided in order to tap a voltage at the Hall sensor when the bending beam is deflected, the voltage rising linearly with the deceleration.